1. Field of the Invention
This invention concerns fluid control valves of the type in which fluid flow through a discharge port controlled by an obturator movable towards and away from the port provides a valve control effect. Such valves often include a control orifice in the fluid flow path upstream of the discharge port, this control orifice being of smaller cross-section than the discharge port so that the pressure of the fluid between the control orifice and the discharge port can be affected by the rate of flow through the discharge port as controlled by the obturator. In this arrangement, the fluid pressure between the discharge port and the control orifice is utilised to actuate a valve member, for instance by being applied to one side of a double-acting piston or diaphragm serving as an actuator for a valve member. The invention will be explained in connection with this arrangement but, as will be discussed, the invention is also applicable to fluid control valves without such a control orifice, i.e. valves in which the obturator provides a simple flow control function, as in a poppet valve.
2. Background Discussion
Typically in fluid control valves of this type the discharge port is formed as an orifice in a generally convex surface facing the obturator, the actual shape of this surface usually resulting from a consideration of manufacturing convenience. However, the shape of this surface affects the relationship between the position of the obturator and the fluid pressure immediately upstream of the discharge port over a small range of positions of the obturator relatively to the discharge port, and that this effect can be utilised to improve the performance of the valve.
If the fluid pressure immediately upstream of the discharge port, for instance between the discharge port and a smaller upstream control orifice, is measured for various positions of the obturator, it is found that this "back pressure" varies between a maximum when the obturator effectively seals the discharge port, and a minimum when the obturator is spaced from the discharge port by a distance equivalent to approximately one-third of the discharge port diameter. If the results are plotted graphically, for most positions of the obturator between these extremes there is a regular relationship between the back pressure and the obturator position: that is to say the plot is essentially a smooth curve. However over a small range of positions the relationship is irregular, giving rise to a "kink" in the curve. The extent of the irregularity depends on the shape of the surface facing the obturator and in which the discharge port is formed. Perhaps fortuitously, the irregularity is least -- that is, the kink is least pronounced -- when the discharge port is provided by a relatively sharp-edged orifice in a convex, e.g. frustoconical, surface facing the obturator, the conventional configuration in this type of valve. See "The Development of some Low Pressure Jet Sensing Techniques" (F.X. Kay) Paper No. 21 at the First European Fluid Power Conference, National Engineering Laboratory, East Kilbride, Scotland, Sept. 1973.